Impossible WHERE for a!=a, a<a, a>a #234
Closed
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
For a table column `a`, the above expressions logically equate to false in all cases. With this patch the optimizer knows about this and queries like: SELECT * FROM t1 WHERE a!=a no longer need to evaluate a!=a for every row. The same applies if the expression was `a<a`, or `a>a` An `EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a` will show: EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a; id select_type table partitions type possible_keys key key_len ref rows filtered Extra 1 SIMPLE NULL NULL NULL NULL NULL NULL NULL NULL NULL Impossible WHERE Similarly `NOT (a!=a)` is always true. EXPLAIN SELECT COUNT(*) FROM t1 WHERE not (a!=a); id select_type table partitions type possible_keys key key_len ref rows filtered Extra 1 SIMPLE NULL NULL NULL NULL NULL NULL NULL NULL NULL Select tables optimized away Warnings: Note 1003 /* select#1 */ select count(0) AS `COUNT(*)` from `test`.`t1` where 1
|
I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it. |
|
Hi, thank you for your contribution. Please confirm this code is submitted under the terms of the OCA (Oracle's Contribution Agreement) you have previously signed by cutting and pasting the following text as a comment: |
|
Hi, thank you for your contribution. Your code has been assigned to an internal queue. Please follow |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
For a table column
a, the above expressions logicallyequate to false in all cases.
With this patch the optimizer knows about this and queries
like:
SELECT * FROM t1 WHERE a!=a
no longer need to evaluate a!=a for every row.
The same applies if the expression was
a<a, ora>aAn
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<awill show:EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a;
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
Similarly
NOT (a!=a)is always true.EXPLAIN SELECT COUNT() FROM t1 WHERE not (a!=a);
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL NULL NULL Select tables optimized away
Warnings:
Note 1003 / select#1 */ select count(0) AS
COUNT(*)fromtest.t1where 1